This application claims priority to France Patent Application No. 98 07 693 filed Jun. 18, 1998.
1. Field of the Invention
The invention concerns a method of controlling safety tripping means in a high voltage generator, more specifically a generator designed for electrostatic application of coating products, the function of the safety tripping means being to stop generation of the high voltage in the event of a risk of discharge in the form of electrical arcing. The invention also concerns a high voltage generator provided with safety tripping means using this method.
2. Description of the Prior Art
During electrostatic application of a coating product, in the course of which particles of the coating product are charged by means of a high voltage generator, it is imperative to detect and eliminate potentially dangerous situations that can lead, in some conditions, to sudden discharges in the form of electrical arcing between the coating product sprayer and the part to be coated, which is generally made of metal and electrically grounded. Prior art safety systems measure the current delivered by the generator at all times, produce at least one current parameter from the measured current and compare that parameter to a threshold. By xe2x80x9ccurrent parameterxe2x80x9d is meant, for example, the value of the current I itself at a given time or its variation (that is to say the time derivative of the current dI/dt). At least these two parameters are preferably taken into account in evaluating the risk of discharge in the form of electrical arcing. If the current parameter is the current itself, it is compared to a maximum current Imax If the current parameter is the current variation (derivative), it is compared to a maximum current variation dI/dtmax. If the current parameter exceeds the preset limit value, a signal or an indication is produced for controlling safety tripping means adapted to disconnect the high voltage or to inhibit the generator in order to prevent electrical arcing.
The authorized values of the energy output by a coating product sprayer in the latest applicable standards are increasingly small, typically 0.24 mJ for liquid coating product and 5 mJ for powder coating product. It is therefore necessary to develop increasingly sensitive and faster safety systems that must also not trip without justification.
A critical situation can arise if the object to be coated and the end of the sprayer at the high voltage suddenly move toward each other. This often occurs during electrostatic application of coating product to objects moving along a conveyor, for example, with the sprayers to the side of the conveyor. It can happen that a projecting part of the object to be painted comes close to the sprayer, creating a risk of discharge in the form of electrical arcing. This can cause a fire, in particular when applying paint containing a solvent.
One example of the problem arises in processing automobile bodyshells. During the coating process, the petrol filler cap cover of the vehicle is left open. If it comes very close to an electrostatic sprayer, the control means associated with the generator detect an abnormal increase in the current, which can lead to tripping out. On resetting, it is standard practice for the safety tripping means to be disabled for a predetermined time interval, while the generator voltage rises. In other words, when the generator voltage begins to rise, the tripping means are inoperative and cannot prevent a discharge in the form of electrical arcing. If the petrol filler cap cover is still near the sprayer during this phase of re-establishing the voltage, a number of electrical arcs can be struck between the coating product sprayer device and the petrol filler cap cover on the bodyshell. In some cases these electrical arcs can cause a fire.
The standardized tests and tests routinely used to assess the behavior of a safety system in regard to a problem of the above type include placing a sphere at a distance of a few centimeters from the end of a sprayer at the high voltage and turning on the high voltage. It is found that most systems currently in use are not able to prevent discharge in the form of electrical arcing with the energy values mentioned above.
The invention overcomes the above difficulties. To be more precise, the invention consists in a method of controlling safety tripping means in a high voltage generator, in particular for electrostatic application of coating product, the tripping means being adapted to interrupt the production of the high voltage in the event of a risk of discharge in the form of electrical arcing, the method consisting in activating the safety tripping means if at least one current parameter reaches a threshold and varying the threshold as a function of the value of the high voltage in accordance with a predetermined variation law.
The current parameter, in the sense defined above, can be the current itself. It can also be the variation of the current (in other words the time derivative of the current dI/dt). The two parameters are preferably used conjointly, a predetermined law of variation as a function of the high voltage value being defined for each of them.
Another noteworthy feature of the invention is that the safety tripping means are inhibited when the high voltage is less than a predetermined minimum value. In other words, the inhibition of the safety systems during the phase in which the voltage rises is no longer set by a time-delay but depends only on whether a minimum value of the high voltage has been reached or not. This feature has an additional advantage. On most generators, the ramp along which the high voltage rises can be adjusted. Consequently, by choosing to enable the safety system as soon as a minimum voltage has been reached, inhibition is rendered independent of adjustment of the voltage ramp.
The invention also consists in a high voltage generator comprising means for generating a high voltage and safety tripping means for disconnecting the high voltage in the event of a risk of discharge in the form of electrical arcing, the generator comprising means for producing a tripping indication varying as a function of the value of the high voltage and associated with a given current parameter and comparator means for comparing the tripping indication to another indication representative of the current parameter itself, the comparator means responding by producing an indication for controlling the safety tripping means.
The generator may advantageously further include comparator means for comparing an indication representative of the high voltage to an indication representative of a minimum voltage, the comparator means producing in response to such comparison an indication controlling means for inhibiting the safety tripping means. The inhibiting means can operate independently of the fact that a tripping threshold can be varied in accordance with the value of the high voltage, as mentioned above.
The high voltage generator as just described, and in particular its safety tripping means, can be implemented using standard electronic circuits performing analog processing of current and voltage signals sampled from the generator or digital processing using a microprocessor, memory circuits and hard-wired logic if the current and voltage signals are first converted to digital data. The term xe2x80x9cindicationxe2x80x9d used in the above definition refers generically and interchangeably to an analog signal processed as such or digital data representing an analog signal.
The invention will be better understood and other advantages of the invention will become more clearly apparent in the light of the following description which is given by way of example only and with reference to the accompanying drawings.